<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>

        function fn() {
            let num = 0
            let yue = +prompt('输入银行卡余额')
            let shis = +prompt('输入当月食宿消费金额')
            let shengh = +prompt('输入当月生活消费金额')
            num = yue - shis - shengh
            return num
        }
        let res = fn()
        console.log(res)
    </script>
</body>

</html>